For conventional helicopters, the tail rotor assembly plays an important part in the aerodynamic operation of the helicopter. The thrust (lift) generated by the tail rotor blades provides the torque to counterbalance the torque effect transmitted to the helicopter fuselage by the main rotor blades. Due to the higher rotational speeds existing outwardly along the outboard segment of the tail rotor blades, the outboard segments of the tail rotor blades provide the major portion of the thrust generated by the tail rotor assembly. This characteristic of tail rotor blades having a rectangular planform is graphically illustrated in FIG. 1 wherein the radius ratio, r/R, defines specific stations along a blade having a total span equal to R. An examination of the lift profile L of FIG. 1 shows that the lift distribution increases to a maximum at about the 0.9 station, and then falls off precipitously.
The inefficient loading distribution existing across the tip section of the outboard blade section, i.e., the lift profile from about the 0.9 station to the 1.0 station, is due primarily to the vortices generated by the tail rotor blades, especially the vortices developed by the tip segments of the tail rotor blades. The three dimensional air flow effects near the tip of the tail rotor blade cause the rapid dropoff outboard of the 0.92 station, as illustrated in FIG. 1. The tip vortices are concentrated vortices that are shed by the lift generating tail rotor blades, the center of each tip vortex intersecting the corresponding trailing tail rotor blade at about the 0.92 station. Just inboard of this station, the interaction of the vortex induces large inflow, and hence, the blade segments in this region, unless designed properly, can stall.
The tail rotor assembly generally provides the major contribution to the noise level of operating helicopters. Of the noise generated by the tail rotor blades, noise arising from blade vortex interaction is significant source of helicopter operating noise. Blade vortex interaction noise arises primarily from interfering tip vortices interacting with the corresponding trailing tail rotor blades. Equally significant in addition to blade vortex interaction noise is high speed impulsive noise generated by the tail rotor blades due to the onset of compressibility effects at the tip segments of the tail rotor blades. The main rotor blades of a helicopter also generate vortices which interact with the tail rotor blades to generate noise.
A need exists to reduce the noise levels resulting from blade vortex interaction and compressibility effects. Preferably, the means utilized to reduce tail rotor blade noise levels does not degrade the thrust efficiency of the tail rotor assembly, i.e., does not require increased power input to maintain the necessary tail rotor assembly thrust.